1. Field of the Invention
The present invention relates to drug-eluting medical devices; more particularly, this invention relates to processes for crimping a polymeric stent to a delivery balloon.
2. Background of the Invention
The art recognizes a variety of factors that affect a polymeric stent's ability to retain its structural integrity when subjected to external loadings, such as crimping and balloon expansion forces. These interactions are complex and the mechanisms of action not fully understand. According to the art, characteristics differentiating a polymeric, bio-absorbable stent scaffold of the type expanded to a deployed state by plastic deformation from a similarly functioning metal stent are many and significant. Indeed, several of the accepted analytic or empirical methods/models used to predict the behavior of metallic stents tend to be unreliable, if not inappropriate, as methods/models for reliably and consistently predicting the highly non-linear behavior of a polymeric load-bearing, or scaffold portion of a balloon-expandable stent. The models are not generally capable of providing an acceptable degree of certainty required for purposes of implanting the stent within a body, or predicting/anticipating the empirical data.
Moreover, it is recognized that the state of the art in medical device-related balloon fabrication, e.g., non-compliant balloons for stent deployment and/or angioplasty, provide only limited information about how a polymeric material might behave when used to support a lumen within a living being via plastic deformation of a network of rings interconnected by struts. In short, methods devised to improve mechanical features of an inflated, thin-walled balloon structure, most analogous to mechanical properties of a pre-loaded membrane when the balloon is inflated and supporting a lumen, simply provides little, if any insight into the behavior of a deployed polymeric stent scaffold. One difference, for example, is the propensity for fracture or cracks to develop in a stent scaffold. The art recognizes the mechanical problem as too different to provide helpful insights, therefore, despite a shared similarity in class of material. At best, the balloon fabrication art provides only general guidance for one seeking to improve characteristics of a balloon-expanded, bio-absorbable polymeric stent.
Polymer material considered for use as a polymeric stent scaffold, e.g. PLLA or PLGA, may be described, through comparison with a metallic material used to form a stent scaffold, in some of the following ways. A suitable polymer has a low strength to weight ratio, which means more material is needed to provide an equivalent mechanical property to that of a metal. Therefore, struts must be made thicker and wider to have the strength needed. The scaffold also tends to be brittle or have limited fracture toughness. The anisotropic and rate-dependant inelastic properties (i.e., strength/stiffness of the material varies depending upon the rate at which the material is deformed) inherent in the material only compound this complexity in working with a polymer, particularly, bio-absorbable polymer such as PLLA or PLGA.
Processing steps performed on, and design changes made to a metal stent that have not typically raised concerns for, or required careful attention to unanticipated changes in the average mechanical properties of the material, therefore, may not also apply to a polymer stent due to the non-linear and sometimes unpredictable nature of the mechanical properties of the polymer under a similar loading condition. It is sometimes the case that one needs to undertake extensive validation before it even becomes possible to predict more generally whether a particular condition is due to one factor or another—e.g., was a defect the result of one or more steps of a fabrication process, or one or more steps in a process that takes place after stent fabrication, e.g., crimping? As a consequence, a change to a fabrication process, post-fabrication process or even relatively minor changes to a stent pattern design must, generally speaking, be investigated more thoroughly than if a metallic material were used instead of the polymer. It follows, therefore, that when choosing among different polymeric stent designs for improvement thereof, there are far less inferences, theories, or systematic methods of discovery available, as a tool for steering one clear of unproductive paths, and towards more productive paths for improvement, than when making changes in a metal stent.
It is recognized, therefore, that, whereas inferences previously accepted in the art for stent validation or feasibility when an isotropic and ductile metallic material was used, such inferences would be inappropriate for a polymeric stent. A change in a polymeric stent pattern may affect not only the stiffness or lumen coverage of the stent in its deployed state supporting a lumen, but also the propensity for fractures to develop when the stent is crimped or being deployed. This means that, in comparison to a metallic stent, there is generally no assumption that can be made as to whether a changed stent pattern may not produce an adverse outcome, or require a significant change in a processing step (e.g., tube forming, laser cutting, crimping, etc.). Simply put, the highly favorable, inherent properties of a metal (generally invariant stress/strain properties with respect to the rate of deformation or the direction of loading, and the material's ductile nature), which simplify the stent fabrication process, allow for inferences to be more easily drawn between a changed stent pattern and/or a processing step and the ability for the stent to be reliably manufactured with the new pattern and without defects when implanted within a living being.
A change in the pattern of the struts and rings of a polymeric stent scaffold that is plastically deformed, both when crimped to, and when later deployed by a balloon, unfortunately, is not as easy to predict as a metal stent. Indeed, it is recognized that unexpected problems may arise in polymer stent fabrication steps as a result of a changed pattern that would not have necessitated any changes if the pattern was instead formed from a metal tube. In contrast to changes in a metallic stent pattern, a change in polymer stent pattern may necessitate other modifications in fabrication steps or post-fabrication processing, such as crimping and sterilization.
One problem encountered with a polymer stent is the stents susceptibility to damage when being crimped to a balloon. Non-uniform forces applied during a crimping process can cause irregular deformations in struts of a polymer stent, which can induce crack formation and loss of strength. There is a continuing need to improve upon the crimping methods, or pre-crimping procedures used for polymer stents to reduce instance of crack formation or irregular strut deformation during stent production.